1. Field of the Invention
This invention relates to a wet friction material that generates a torque by applying a high pressure to an opposite surface while being immersed in oil. In detail, this invention relates to a segment-type friction material made by joining friction material substrates that were cut into a segment piece onto one or both surfaces of a core metal of a flat ring shape along an entire circumference thereof with an adhesive. Otherwise, this invention relates to a ring-shaped friction material made by joining ring-shaped friction material substrates onto one or both surfaces of a core metal of a flat ring shape thereof with an adhesive.
2. Description of the Related Art
In recent years, for the purpose of reducing a shift-shock by multistage of an automatic transmission, which may be referred to as “AT” hereafter, of an automobile or the like, it is required to improve a shift transmission feature (engagement/disengagement feature) in a wet friction material which is used for AT. To improve a disengagement feature, it is effective to make it harder for an oil film which is cased by an automatic transmission fluid, which may be referred to as “ATF” hereafter, to generate between a friction surface of a wet friction material and a counterpart plate in a disengaged state by making a pore diameter of the wet friction material large or by increasing a porosity. (“ATF” is a registered trademark of Idemitsu Kosan Co., Ltd.) According to an invention described in Japanese Laid Open Patent Publication No. 2004-138121, there is provided a wet friction material that it includes silica of an average particle size 1 μm to 10 μm and a disk-shaped diatom earth in paper substrates. Also, this wet friction material uses a hardened material of liquid resin composition that can be obtained by mixing resol-type phenolic resin and silicone resin as a bounding material. Accordingly, it can obtain a wet friction material that is greater in heat resistance (resistant heat spot) and is improved a positive μ-V slope characteristic while having a high friction coefficient.
However, in the wet friction material described in the above Japanese Laid Open Patent Publication No. 2004-138121, it contains diatom earth that an absolute specific gravity is low, which is 25% by weight to 45% by weight of paper substrates, and inter-fiber pores of paper substrates are filled in with the diatom earth. Therefore, a pore diameter becomes small and there is a possibility that it can not obtain a preferable disengagement feature. In contrast, if it increases fibrillation of fibers of paper substrates or decreases lining density so as to increase pore diameter or porosity, there is a problem that it generates negative effects such as strength reduction or increase in settling quantity etc.
Then, an object of this invention is to provide a wet friction material that it is greater in a disengagement feature by increasing a pore diameter of a wet friction material as well as reducing a drag torque, while it is greater in a positive μ-V slope characteristic without generating such negative effects, strength reduction or increase in settling quantity etc.